A fully defined medium has been devised which supports organogenesis of embryonic pancreatic rudiments in vitro, temporally paralleling morphologic and enzymatic differentiation in vivo. When rudiments are cultured in roller tubes with complete medium, prolonged growth (10 weeks) is feasible with periodic subdivision of the tissue. This yields derived tissue with normal organ-cellular architecture and the enzymatic profile of fully differentiated tissue. The feasibility of prolonged culture and demonstrated susceptibility of cells to transformation by chemical carcinogens has indicated this to be ideal for obtaining normal or abnormal responses to exogenous agents. It is proposed to utilize this technique for determining the responses to trophic agents and anti-proteases.